1. Field of Invention
The present invention generally relates to an oscillator and an oscillation signal generation method, and more particularly, to a multi-phase oscillator and a multi-phase oscillation signal generation method.
2. Description of Related Art
The oscillator plays a very important role in both communication systems and digital circuits now. In digital circuits, the oscillator provides multi-phase clock signals to the system. In communication systems, the multi-phase oscillation output of the oscillator can be used as a carrier in the modulation operation or as a local oscillator in the demodulation operation.
Generally speaking, there are two major methods to design the oscillator: (1) using the LC charge-discharge circuit that consists of an inductor (L) and a capacitor (C); (2) connecting an odd number of the inverters in serial to generate an oscillation signal by the contribution of the time delay in between the inverters. Recently, along with the great progress of the CMOS manufacture process, and also considering of the system integrity, the design method that includes the inductor elements is seldom used because the Q value cannot be increased and a great area is required for the on-chip inductor. The researches of the multi-phase output method mostly concentrate on the study of the outputs have a 180° or a 90° phase difference.
The configuration having a 180° phase difference is shown in FIG. 1, FIG. 1 schematically shows a circuit diagram of a conventional regeneration ring oscillator (please refer to T. Kwasniewski et al., “Inductorless oscillator design for personal communication device a 1.2 um CMOS process case study,” Proc. of IEEE Custom Integrated Circuit Conference, pp 327-330, 1995 and M. Thamsirianunt and T. A. Kwasniewski, “CMOS VCO”s for PLL frequency synthesis in GHz digital mobile radio communications,” IEEE journal of Solid-State Circuits, vol. 32, no. 10, pp. 1511-1524, October 1997.). Each of the inverter latches using a two-stage regenerator 102, 104, is connected with the ring oscillator that consists of two sets of the three-stage inverters (such as 106, 108, 110 and 112, 114, 116 as shown in FIG. 1) that are coupled each other to generate the oscillation signals. The two-stage regenerators 102 and 104 of the present configuration provide the energy to the ring oscillator that consists of the three-stage inverters that consists of the inverters connected in series, so that the oscillator 100 obtains a lower phase noise. However, the more elements it uses result in greater time delay and larger power consumption.
The first configuration having a 90° phase difference is shown in FIG. 2, FIG. 2 schematically shows a circuit diagram of a conventional RC-CR oscillator having a 90° phase difference (please refer to Qiuting Huang, “The design of a direct-conversion paging receiver quadrature converter for wrist watch application electronics,” Proc. of IEEE International Conference on Circuits and Systems, vol.3, pp 29-32, 1998.). It is a system configuration consists of a single resistor capacitor-capacitor resistor (RC-CR). Although the RC-CR configuration can achieve the function of outputting a 90° phase difference signal, since each of the RC-CR in the configuration is coupled to the input nodes of the differential configuration, the distortion is easily occurred due to the mismatch of the input nodes.
The second configuration having a 90° phase difference is shown in FIG. 3. FIG. 3 schematically shows a circuit diagram of a conventional ring oscillator that generates a 90° phase difference (please refer to Chung-Yu Wu and Hong-Sing Kao, “A 1.8 GHz CMOS quadrature voltage-controlled oscillator (VCO) using the constant-current LC ring oscillator structure,” Proceedings of the 1998 IEEE International Symposium on Circuits and Systems vol. 4, pp. 378-381, 1998.). It utilizes a two-stage LC tank that consists of an inductor and a capacitor connected in serial and the negative resistance circuit to obtain the oscillation signals on the nodes I and Q.
The third configuration having a 90° phase difference is shown in FIG. 4. FIG. 4 schematically shows a circuit diagram of a conventional ring oscillator that consists of a six-stage differential amplifier including six differential amplifiers connected in serial (please refer to Y. Sugimoto and T. Ueno, “The design of 1 V, 1 GHz CMOS VCO circuit with in-phase and quadrature-phase output,” Proceedings of IEEE International Symposium on Circuits and Systems vol.1, pp. 269-272, 1997.). It utilizes three currents that are generated by the first, the third and the fifth differential amplifiers to generate the outputs having a 90 phase difference, and to obtain the expected oscillation signals on the nodes I and Q. However, the present method easily results in an oscillation signal error due to the mismatch of the three output currents that have a 120° phase difference.
FIG. 5 schematically shows a circuit diagram of a conventional ring oscillator that consists of a four-stage differential amplifier (please refer to U.S. Pat. No. 5,635,880). It utilizes the microwave theory of the differential amplifier to appropriately adjust the circuit bias. In this circuit, the Miller effect is used to constitute a RC loop to configure a ring oscillator circuit that consists of a four-stage differential amplifier to generate multi-phase oscillation signals.
FIG. 6 schematically shows a circuit diagram of a conventional ring oscillator that consists of branch feedback loops (please refer to U.S. Pat. No. 6,075,419). The proper branch feedback loops are added to the ring oscillator consisting of the inverters that are connected in serial, so that the output signals are evenly feedback applied to each input node of the serially connected oscillator to generate the multi-phase oscillator signals.
FIG. 7 schematically shows a circuit diagram of the conventional injection phase locked oscillation signal generator (please refer to U.S. Pat. No. 6,188,291). It uses two or more than two serially connected ring oscillators having the same time period to generate and output the signals. The total phase 360° is formed by the serially connected ring oscillator that generates a phase difference having the same degree. It is based on the theory that a set of the oscillation signals is injected to lock another set of the signals, so that the expected phase difference can be obtained efficiently. Moreover, the present configuration also reduces the phase noise by adding a periodic low noise oscillation signal source 702.
The cases shown in FIG. 5 through FIG. 7 mentioned above are all the multi-phase oscillators. However, they can only provide the oscillation signals with limited varieties of the phase differences.